The invention relates to a connecting pin for fastening dental crown or jaw segments.
Printed publication DE 3,807,591 C2 discloses a socket connection for connecting work models of individual dental crown or jaw segments to a model base. The socket connection comprises a connecting pin and a guiding sleeve which is rigidly connected to the model base. The connecting pin bearing the work models of the segments is releasably inserted into the guiding sleeve in the model base, as a result of which the work models are held securely on the model base for precision-working; in addition, the individual segments can be removed again at any time from the model base.
The connecting pin consists of two shanks, namely a fixing shank for permanent fixing in the underside of an individual tooth or jaw segment and a socket shank for releasable mounting in the guiding sleeve inserted into the model base. The fixing shank is inserted into a cylindrical hole which has been bored in the hardened plaster of the work model of the tooth or jaw segment, and it is connected permanently to the model by means of adhesive. The surface area of the fixing shank has circular grooves which provide for additional strengthening of the join between connecting pin and model.
A problem in this regard is that, on the one hand, the fixing shank and the borehole in the model have to be produced with the least possible tolerance and with little play, so as to ensure the greatest possible precision for working the model. On the other hand, because of this production free of play, there is a risk that the fixing shank, upon introduction into the borehole in the model, will tilt so that the axes of fixing shank and borehole are no longer parallel and, on account of the inclined position of insertion of the fixing shank, the model will not be held in the intended manner relative to the model base. Moreover, the obliquely inserted fixing shank cannot be sunk completely into the borehole, with the result that adjacent dental crown or jaw segments are placed slightly offset in height in the model base, which negatively affects the working precision of the model.
A further problem lies in the nature of the connection between the socket shank of the connecting pin and the guiding sleeve in the model base. The socket shank comprises two sections, of which at least one is of cylindrical design and is fitted releasably into the guiding sleeve of complementary shape. Between the cylindrical surface area of the socket shank and the cylindrical inner surface of the guiding sleeve, high static friction forces arise on account of the large friction surfaces, and these forces have to be overcome when removing a segment. If, after completion of the model work, the connecting pin is to be removed from the guiding sleeve, the work model may be damaged when releasing the socket shank from the sleeve, for example there may be undesired detachment of the connecting pin from the permanent fixture in the borehole of the model. The model base, too, may be damaged by the high release forces which are required for removing the work model.
Comparable socket connections are also disclosed by printed publications DE 3,301,617 A1 and DE 3,108,700 A1.
Furthermore, it is known from DE 196 00 854 to introduce a longitudinal slot into the fixing shank of the connecting pin in order to permit an elastic compression of the shank parts for easier insertion of the fixing shank into the work model. After insertion, the shank parts tend radially outward and generate a clamping force which fixes the connecting pin and by means of which the connecting pin is held securely in the work model.
The invention is based on the problem of making available a connection, which is inexpensive and easy to produce, between a model base and a work model of a dental crown or jaw segment, which connection permits precision-working of a dental work model and can be easily removed again.
According to the invention, this problem is achieved by running the shape of the surface area of the fixing shank to be connected to the work model. As the surface area of the fixing shank, not loaded by external forces, deviates from the cylinder shape and as the surface area in the region of the smallest radius is radially set back relative to the cylinder shape, an undercut is formed in the radial direction. When there are no external forces applied on the fixing shank, the shank parts in the unloaded state tend to run slightly radially outward from their attachment foot; the undercut is here defined as the difference between the maximum radius and the minimum radius of the fixing shank surface area in the unloaded state. The slot extends with a defined slot thickness between the shank parts, said shank parts being designed to be flexible at least to the extent that the shank parts in the area of the free end face of the fixing shank can be compressed for insertion into the borehole in the work model.
As a result of the compression of the shank parts, the surface area of the fixing shank changes and is now approximately cylindrical. Because of the elasticity and inherent stress, the compressed shank parts press radially outward. The surface area is approximately cylindrical, which means that, with a connecting pin fitted into the borehole of the work model, a surface pressure is exerted on the inner wall of the borehole, and this surface pressure is approximately uniform along the axial length of the fixing shank. This results in high static friction forces which securely clamp the fixing shank in the borehole, but which are distributed uniformly over the inner surface of the borehole so that force peaks exerting a load on the borehole are avoided. In addition, insertion into the borehole is made easier because the diameter of the fixing shank, with the shank parts compressed, is approximately the same as or slightly smaller than the diameter of the borehole, so that there is only minimal friction to be overcome upon insertion. The diameter of the cylindrical surface area here corresponds to the diameter of the unloaded fixing shank in the area of the smallest radius.
The undercut of the unloaded fixing shank, in combination with the slot formed between the shank parts, ensures that the shank parts can be compressed until a cylindrical surface area of the fixing shank is obtained.
Attainment of the cylinder shape is preferably assisted by the fact that the distance between the shank parts in the area of the free end face of the fixing shankxe2x80x94the slot thickness in this areaxe2x80x94is at least as great as the sum of the two radially opposite undercuts in the area of the attachment foot of the shank parts or in the area of a support shoulder which is advantageously formed at the transition between fixing shank and socket shank. The undercuts measured in the radial direction are leveled out by the compression, in which respect, with uniform pressure on the fixing shank, each shank part levels out its radial undercut until the surface area of the fixing shank assumes a cylinder shape.
The fixing shank can be designed with a uniform slot thickness which results in a non-uniform thickness of the shank parts, as viewed along the axial length, so that an undercut is formed at the foot of the shank parts. However, it may also be expedient to design the shank parts uniformly and to provide the slot with a varying slot thickness.
In one advantageous embodiment, between the socket shank of the connecting pin, projecting into the guiding sleeve, and the guiding sleeve itself there is at least one cone-shaped connecting section whose surface cooperates with the outer surface area of the socket shank and with the inner surface of the guiding sleeve, the axial length of the connecting section being at most half the length of the socket shank. The connecting section is used for guiding the connecting pin in the sleeve, and in this section the contacting surfaces of the socket shank and of the sleeve are of complementary design and lie against one another with static friction. Adjoining the conical connecting section there is a further section, and in this section the radially facing surfaces of socket shank and sleeve are either of non-complementary design or have concentric, spaced-apart surface areas, so that in any case the inner surface in the guiding sleeve and the outer surface area of the socket shank lie at a distance from one another and are therefore free from friction. This ensures that the friction surface can be essentially limited only to the first conically shaped section of reduced length. The frictional forces, in particular static friction forces, are reduced to such an extent that, in order to release the socket connection, it is necessary to apply only slight reaction forces which do not damage the work model. The frictional forces should in particular not exceed those holding forces with which the fixing shank is held in the borehole of the work model. At the same time, however, the holding forces of the connection are sufficient to exclude the possibility of play developing. The connection is able to take up, without movement, the forces which normally occur in model working.
It may be expedient to extend the axial length of the conical connecting section to as much as two thirds of the axial length of the socket shank.
The limited length of the conical section also makes it possible, in a preferred development, to provide the socket shank with a second conical connecting section which is arranged at an axial distance from the first conical connecting section and to which there is assigned a section of once again complementary shape in the guiding sleeve. Provided between the two connecting sections, there is a middle section which performs no connecting function. In the area of this middle section, which is preferably of cylindrical design, the socket shank and the guiding sleeve are of non-complementary design or lie at a distance from one another, in particular without any contact between the surface area of the socket shank and the inner surface of the guiding sleeve.
In a preferred embodiment, the socket shank is provided with three sections in the order of conical section/cylindrical section/conical section, where the conical sections assume the function of connecting sections, to which there are assigned sections of complementary shape in the guiding sleeve, while the cylindrical section by contrast is arranged at a distance from the inner surface of the guiding sleeve. In this embodiment, the total axial length of the socket shank can be used for the connection to the guiding sleeve, the first conical section being arranged adjoining the support shoulder of the connecting pin, and the second conical section being arranged in the area of the free end face of the socket shank, so that the pin is guided in the sleeve at both axially opposite ends of the socket shank. The long guide ensures an optimum seat in the sleeve, and the reduced connecting surfaces ensure easy release of the connection.
The axial length of all the conical sections taken together is preferably at most two thirds, particularly at most half, of the length of the socket shank.